Polyimide-based composite film and display device comprising same

ABSTRACT

Embodiments relate to a polyimide-based composite film, which comprises a base film comprising a polyimide-based resin; and a functional layer disposed on the base film, wherein when the side of the functional layer located opposite to the side in contact with the base film is referred to as a first side and when the side of the base film located opposite to the side in contact with the functional layer is referred to as a second side, the adhesiveness index represented by Equation 1 is 3.5 or less.

TECHNICAL FIELD

Embodiments relate to a polyimide-based composite film that iscolorless, transparent, and excellent in mechanical properties andoptical properties, and a display device comprising the same.

BACKGROUND ART OF THE INVENTION

Polyimide-based resins such as polyamide-imide (PAT) are excellent inresistance to friction, heat, and chemicals. Thus, they are employed insuch applications as primary electrical insulation, coatings, adhesives,resins for extrusion, heat-resistant paintings, heat-resistant boards,heat-resistant adhesives, heat-resistant fibers, and heat-resistantfilms.

Such a polyimide-based resin is used in various fields. For example,polyimide-based resins are made in the form of a powder and used as acoating for a metal or a magnetic wire. They are mixed with otheradditives depending on the applications thereof.

In addition, polyimide-based resins are used together with afluoropolymer as a painter for decoration and corrosion prevention. Theyalso play a role of bonding a fluoropolymer to a metal substrate. Inaddition, a polyimide-based resin is used to coat kitchenware, used as amembrane for gas separation by virtue of its heat resistance andchemical resistance, and used in natural gas wells for filtration ofsuch contaminants as carbon dioxide, hydrogen sulfide, and impurities.

In recent years, a polyimide-based resin has been developed in the formof a film, which is less expensive and has excellent optical,mechanical, and thermal characteristics.

DISCLOSURE OF THE INVENTION Problem to be Solved

Embodiments aim to provide a polyimide-based composite film that iscolorless, transparent, and excellent in mechanical properties andoptical properties, and a display device comprising the same.

Solution to the Problem

The polyimide-based composite film according to an embodiment comprisesa base film comprising a polyimide-based resin; and a functional layerdisposed on the base film, wherein when the side of the functional layerlocated opposite to the side in contact with the base film is referredto as a first side and when the side of the base film located oppositeto the side in contact with the functional layer is referred to as asecond side, the adhesiveness index represented by the followingEquation 1 is 3.5 or less.

$\begin{matrix}{{{Adhesiveness}\mspace{14mu} {index}} = \frac{{Fc} \times {Ec}}{Sc}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, Sc is (Sz1+Sz2)/2 μm, Fc is (F1+F2)/0.6, and Ec is (E1+E2)/40dyne/cm, wherein Sz1 is the Sz roughness (μm) of the first side, Sz2 isthe Sz roughness (μm) of the second side, F1 is the coefficient ofstatic friction between the first side and the second side, F2 is acoefficient of kinetic friction between the first side and the secondside, E1 is the surface energy (dyne/cm) of the first side, and the E2is the surface energy (dyne/cm) of the second side.

The display device according to another embodiment comprises a displaypanel; and a cover window disposed on the display panel, wherein thecover window comprises a base film and a functional layer disposed onthe base film, the base film comprises a polyimide-based resin, and whenthe side of the functional layer located opposite to the side in contactwith the base film is referred to as a first side and when the side ofthe base film located opposite to the side in contact with thefunctional layer is referred to as a second side, the adhesiveness indexrepresented by the above Equation 1 is 3.5 or less.

Advantageous Effects of the Invention

The polyimide-based composite film according to an embodiment has anappropriate range of the surface roughness, coefficients of friction,and surface energies of the first side and the second side, and thevalue of Equation 1 satisfies 3.5 or less. Thus, the slip properties ofthe first side and second side can be enhanced.

As a result, the polyimide-based composite film according to anembodiment can effectively prevent the problem caused by the first sideand the second side being excessively adhered to each other. The film iscolorless and transparent with enhanced mechanical properties andoptical properties such as haze, yellow index, and modulus.

The display device according to another embodiment also may haveenhanced mechanical properties and optical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a polyimide-based composite filmaccording to an embodiment.

FIG. 2 is a cross-sectional view of a display device according to anembodiment.

FIG. 3 is a schematic flow diagram of a process for preparing a basefilm according to an embodiment.

FIG. 4 schematically illustrates process facilities for preparing a basefilm according to an embodiment.

DETAILED DESCRIPTION FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail withreference to embodiments. The embodiments are not limited to thosedescribed below. Rather, they can be modified into various forms as longas the gist of the invention is not altered.

Throughout the present specification, in the case where each film,window, panel, layer, or the like is mentioned to be formed “on” or“under” another film, window, panel, layer, or the like, it means notonly that one element is directly formed on or under another element,but also that one element is indirectly formed on or under anotherelement with other element(s) interposed between them. In addition, theterm on or under with respect to each element may be referenced to thedrawings. For the sake of description, the sizes of individual elementsin the appended drawings may be exaggeratedly depicted and do notindicate the actual sizes.

In this specification, when a part is referred to as “comprising” anelement, it is to be understood that the part may comprise otherelements as well, unless otherwise indicated.

All numbers and expressions relating to quantities of components,reaction conditions, and the like used herein are to be understood asbeing modified by the term “about” unless specifically stated otherwise.

The terms first, second, and the like are used herein to describevarious elements, and the elements should not be limited by the terms.The terms are used only for the purpose of distinguishing one elementfrom another.

In addition, the term “substituted” as used herein means to besubstituted with at least one substituent group selected from the groupconsisting of deuterium, —F, —Cl, —Br, —I, a hydroxyl group, a cyanogroup, a nitro group, an amino group, an amidino group, a hydrazinegroup, a hydrazone group, an ester group, a ketone group, a carboxylgroup, a substituted or unsubstituted alkyl group, a substituted orunsubstituted alkenyl group, a substituted or unsubstituted alkynylgroup, a substituted or unsubstituted alkoxy group, a substituted orunsubstituted alicyclic organic group, a substituted or unsubstitutedheterocyclic group, a substituted or unsubstituted aryl group, and asubstituted or unsubstituted heteroaryl group. The substituent groupsenumerated above may be connected to each other to form a ring.

Polyimide-Based Composite Film

Referring to FIG. 1, the polyimide-based composite film according to anembodiment comprises a base film (110) and a functional layer (120)disposed on the base film (110).

The base film (110) may be a support layer that supports the functionallayer (120). In addition, the base film (110) may comprise apolyimide-based resin. For example, the base film (110) may be apolyimide-based film.

The functional layer (120) may be formed as a coating on the base film(110). The functional layer (120) may be laminated on the base film(110). The functional layer (120) may be bonded on the base film (110).

The functional layer (120) may be a coating layer coated on the basefilm (110). The functional layer (120) may comprise a curable resin.Specifically, the functional layer (120) may be a curable coating layer.

The functional layer (120) may function to enhance the mechanicalproperties and/or optical properties of the base film (110). Thefunctional layer may comprise an antireflection layer, an antifoulinglayer, a hard-coating layer, and a scratch-resistant layer.

As shown in FIG. 1, the functional layer (120) comprises a first side(101). The first side (101) is a side opposite to the side of thefunctional layer (120) on which the base film (110) is disposed. Thefirst side (101) is a side located opposite to the side of thefunctional layer (120) in contact with the base film (110). The firstside (101) may be the upper side of the functional layer (120). Forexample, the first side (101) may be the top side of the functionallayer (120).

The base film (110) comprises a second side (102). The second side (102)is a side opposite to the side of the base film (110) on which thefunctional layer (120) is disposed. The second side (102) is a sidelocated opposite to the side of the base film (110) in contact with thefunctional layer (120). The second side (102) may be the lower side ofthe base film (110). For example, the second side (102) may be thebottom side of the base film (110).

In the polyimide-based composite film according to an embodiment, thefirst side (101) and the second side (102) are opposed to each other. Inaddition, in the polyimide-based composite film according to anembodiment, the first side (101) and the second side (102) are disposedat the top and bottom, respectively. For example, the first side (101)may be the top side of the polyimide-based composite film according toan embodiment, and the second side (102) may be the bottom side of thepolyimide-based composite film according to an embodiment. Accordingly,when the polyimide-based composite film according to an embodiment iswound, the first side and the second side may be in direct contact witheach other.

Base Film (110)

The base film (110) according to an embodiment comprises apolyimide-based resin. The base film (110) may further comprise afiller. For example, the base film (110) may comprise a polyimide-basedresin and a filler.

The polyimide-based resin may be prepared by simultaneously orsequentially reacting reactants that comprise a diamine compound and adianhydride compound. Specifically, the polyimide-based resin maycomprise a polyimide polymer prepared by polymerizing a diamine compoundand a dianhydride compound.

In addition, the polyimide-based resin may comprise a polyamide-imidepolymer that contains an imide repeat unit derived from thepolymerization of a diamine compound and a dianhydride compound and anamide repeat unit derived from the polymerization of a diamine compoundand a dicarbonyl compound.

Since the polyamide-imide polymer contains an imide repeat unit, it mayfall under a polyimide-based resin in a broad sense.

The diamine compound is a compound that forms an imide bond with thedianhydride compound and forms an amide bond with the dicarbonylcompound, to thereby form a copolymer.

The diamine compound is not particularly limited, but it may be, forexample, an aromatic diamine compound that contains an aromaticstructure. For example, the diamine compound may be a compoundrepresented by the following Formula 1.

H₂N-(E)_(e)-NH₂  [Formula 1]

In Formula 1, E may be selected from a substituted or unsubstituteddivalent C₆-C₃₀ aliphatic cyclic group, a substituted or unsubstituteddivalent C₄-C₃₀ heteroaliphatic cyclic group, a substituted orunsubstituted divalent C₆-C₃₀ aromatic cyclic group, a substituted orunsubstituted divalent C₄-C₃₀ heteroaromatic cyclic group, a substitutedor unsubstituted C₁-C₃₀ alkylene group, a substituted or unsubstitutedC₂-C₃₀ alkenylene group, a substituted or unsubstituted C₂-C₃₀alkynylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O)₂—, —Si(CH)₂—,—C(CH₃)₂—, and —C(CF₃)₂—.

e is selected from integers of 1 to 5. When e is 2 or more, the Es maybe the same as, or different from, each other.

-   -   (E)_(e) in Formula 1 may be selected from the groups represented        by the following Formulae 1-1a to 1-14a, but it is not limited        thereto.

Specifically, (E)_(e) in the above Formula 1 may be selected from thegroups represented by the following Formulae 1-1b to 1-13b, but itis notlimited thereto.

More specifically, (E)_(e) in the above Formula 1 may be the grouprepresented by the above Formula 1-6b.

In an embodiment, the diamine compound may comprise a compound having afluorine-containing substituent. Alternatively, the diamine compound maybe composed of a compound having a fluorine-containing substituent. Insuch event, the fluorine-containing substituent may be a fluorinatedhydrocarbon group and specifically may be a trifluoromethyl group. Butit is not limited thereto.

In another embodiment, one kind of diamine compound may be used as thediamine compound. That is, the diamine compound may be composed of asingle component.

For example, the diamine compound may comprise2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) represented by thefollowing formula, but it is not limited thereto.

The dianhydride compound has a low birefringence value, so that it cancontribute to enhancements in the optical properties such astransmittance of a film that comprises the polyimide-based resin.

The dianhydride compound is not particularly limited, but it may be, forexample, an aromatic dianhydride compound that contains an aromaticstructure. For example, the aromatic dianhydride compound may be acompound represented by the following Formula 2.

In Formula 2, G may be a group selected from a substituted orunsubstituted tetravalent C₆-C₃₀ aliphatic cyclic group, a substitutedor unsubstituted tetravalent C₄-C₃₀ heteroaliphatic cyclic group, asubstituted or unsubstituted tetravalent C₆-C₃₀ aromatic cyclic group,or a substituted or unsubstituted tetravalent C₄-C₃ heteroaromaticcyclic group, wherein the aliphatic cyclic group, the heteroaliphaticcyclic group, the aromatic cyclic group, or the heteroaromatic cyclicgroup may be present alone, may be bonded to each other to form acondensed ring, or may be bonded by a bonding group selected from asubstituted or unsubstituted C₁-C₃₀ alkylene group, a substituted orunsubstituted C₂-C₃₀ alkenylene group, a substituted or unsubstitutedC₂-C₃ alkynylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O)₂—,—Si(CH₃)₂—, —C(CH₃)₂—, and —C(CF₃)₂—.

G in the above Formula 2 may be selected from the groups represented bythe following Formulae 2-1a to 2-9a, but it is not limited thereto.

Fore example, G in the above Formula 2 may be the group represented bythe above Formula 2-8a.

In an embodiment, the dianhydride compound may comprise a compoundhaving a fluorine-containing substituent. Alternatively, the dianhydridecompound may be composed of a compound having a fluorine-containingsubstituent. In such event, the fluorine-containing substituent may be afluorinated hydrocarbon group and specifically may be a trifluoromethylgroup. But it is not limited thereto.

In another embodiment, the dianhydride compound may be composed of asingle component or a mixture of two components.

For example, the dianhydride compound may comprise2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA)represented by the following formula, but it is not limited thereto.

The diamine compound and the dianhydride compound may be polymerized toform a polyamic acid.

Subsequently, the polyamic acid may be converted to a polyimide througha dehydration reaction, and the polyimide comprises an imide repeatunit.

The polyimide may form a repeat unit represented by the followingFormula A.

In Formula A, E, G, and e are as described above.

For example, the polyimide may comprise a repeat unit represented by thefollowing Formula A-1, but it is not limited thereto.

In Formula A-1, n is an integer of 1 to 400.

The dicarbonyl compound is not particularly limited, but it may be, forexample, a compound represented by the following Formula 3.

In Formula 3, J may be selected from a substituted or unsubstituteddivalent C₆-C₃₀ aliphatic cyclic group, a substituted or unsubstituteddivalent C₄-C₃₀ heteroaliphatic cyclic group, a substituted orunsubstituted divalent C₆-C₃₀ aromatic cyclic group, a substituted orunsubstituted divalent C₄-C₃₀ heteroaromatic cyclic group, a substitutedor unsubstituted C₁-C₃₀ alkylene group, a substituted or unsubstitutedC₂-C₃₀ alkenylene group, a substituted or unsubstituted C₂-C₃₀alkynylene group, —O—, —S—, —C(═O)—, —CH(OH)—, —S(═O)₂—, —Si(CH₃)₂—,—C(CH₃)₂—, and —C(CF₃)₂—.

j is selected from integers of 1 to 5. When j is 2 or more, the Js maybe the same as, or different from, each other.

X is a halogen atom. Specifically, X may be F, Cl, Br, I, or the like.More specifically, X may be C₁, but it is not limited thereto.

(J)_(j) in the above Formula 3 may be selected from the groupsrepresented by the following Formulae 3-1a to 3-14a, but it is notlimited thereto.

Specifically, (J)_(j) in the above Formula 3 may be selected from thegroups represented by the following Formulae 3-1b to 3-8b, but it is notlimited thereto.

More specifically, (J)_(j) in the above Formula 3 may be the grouprepresented by the above Formula 3-1b, the group represented by theabove Formula 3-2b, or the group represented by the above Formula 3-3b.

In an embodiment, a mixture of at least two kinds of dicarbonylcompounds different from each other may be used as the dicarbonylcompound. If two or more dicarbonyl compounds are used, at least twodicarbonyl compounds in which (J)_(j) in the above Formula 2 is selectedfrom the groups represented by the above Formulae 3-1b to 3-8b may beused as the dicarbonyl compound.

In another embodiment, the dicarbonyl compound may be an aromaticdicarbonyl compound that contains an aromatic structure.

For example, the dicarbonyl compound may comprise a first dicarbonylcompound and/or a second dicarbonyl compound.

The first dicarbonyl compound and the second dicarbonyl compound may bean aromatic dicarbonyl compound, respectively.

The first dicarbonyl compound and the second dicarbonyl compound may becompounds different from each other.

For example, the first dicarbonyl compound and the second dicarbonylcompound may be aromatic dicarbonyl compounds different from each other,but they are not limited thereto.

If the first dicarbonyl compound and the second dicarbonyl compound arean aromatic dicarbonyl compound, respectively, they comprise a benzenering. Thus, they can contribute to improvements in the mechanicalproperties such as surface hardness and tensile strength of a film thusproduced that comprises the polyamide-imide resin.

The dicarbonyl compound may comprise terephthaloyl chloride (TPC),isophthaloyl chloride (IPC), and 1,1′-biphenyl-4,4′-dicarbonyldichloride (BPDC), as represented by the following formulae, or acombination thereof. But it is not limited thereto.

For example, the first dicarbonyl compound may comprise BPDC, and thesecond dicarbonyl compound may comprise TPC, but they are not limitedthereto.

Specifically, if BPDC is used as the first dicarbonyl compound and TPCis used as the second dicarbonyl compound in a proper combination, afilm that comprises the polyamide-imide resin thus produced may havehigh oxidation resistance.

Alternatively, the first dicarbonyl compound may comprise IPC(isophthaloyl chloride), and the second dicarbonyl compound may compriseTPC, but they are not limited thereto.

Specifically, if IPC is used as the first dicarbonyl compound and TPC isused as the second dicarbonyl compound in a proper combination, a filmthat comprises the polyamide-imide resin thus produced may not only havehigh oxidation resistance, but is also economical since the costs can bereduced.

The diamine compound and the dicarbonyl compound may be polymerized toform a repeat unit represented by the following Formula B.

In Formula B, E, J, e, and j are as described above.

For example, the diamine compound and the dicarbonyl compound may bepolymerized to form amide repeat units represented by the followingFormulae B-1 and B-2.

In Formula B-1, x is an integer of 1 to 400.

In Formula B-2, y is an integer of 1 to 400.

The filler may be at least one selected from the group consisting ofbarium sulfate, silica, and calcium carbonate. As the base filmcomprises the filler, it is possible to enhance not only the roughnessand winderability but also the effect of improving the scratches causedby sliding in the preparation of the film.

In addition, the filler may have a particle diameter of 0.01 μm to lessthan 1.0 μm. For example, the particle diameter of the filler may be0.05 μm to 0.9 μm or 0.1 μm to 0.8 μm, but it is not limited thereto.

The base film may comprise the filler in an amount of 0.01 to 3% byweight based on the total weight of the base film. For example, the basefilm may comprise the filler in an amount of 0.05 to 2.5% by weight, 0.1to 2% by weight, or 0.2 to 1.7% by weight, based on the total weight ofthe base film, but it is not limited thereto.

In an embodiment, the Sz1 may be 0.01 μm to 10 μm, and the Sz2 may be0.01 μm to 10 μm. The Sz1 may be 0.01 μm to 5 μm, and the Sz2 may be0.01 μm to 5 μm.

Specifically, the Sz1 may be 0.01 μm to 4 μm, 0.01 μm to 3 μm, 0.05 μmto 5 μm, 0.05 μm to 4 μm, 0.05 μm to 3 μm, or 0.1 μm to 3 μm, but it isnot limited thereto.

In addition, the Sz2 may be 0.01 μm to 4 μm, 0.01 μm to 3 μm, 0.05 μm to5 μm, 0.05 μm to 4 μm, 0.05 μm to 3 μm, 0.05 μm to 2 μm, or 0.1 μm to 2μm, but it is not limited thereto.

For example, the Sz1 may be 0.01 μm to 4 μm, and the Sz2 may be 0.01 μmto 4 μm. The Sz1 may be 0.01 μm to 3 μm, and the Sz2 may be 0.01 μm to 3μm. The Sz1 may be 0.02 μm to 3 μm, and the Sz2 may be 0.02 μm to 3 μm.The Sz1 may be 0.03 μm to 2 μm, and the Sz2 may be 0.03 μm to 2 μm. TheSz1 may be 0.1 μm to 1 μm, and the Sz2 may be 0.1 μm to 2 μm.

According to an embodiment, the F1 is 0.1 to 0.8, and the F2 is 0.1 to0.8. For example, the F1 may be 0.1 to 0.7, 0.2 to 0.6, 0.3 to 0.6, 0.4to 0.6, or 0.4 to 0.5. The F2 may be 0.1 to 0.7, 0.1 to 0.6, 0.1 to 0.5,0.1 to 0.4, 0.1 to 0.3, or 0.1 to 0.2.

According to an embodiment, the E1 may be 10 to 100 dyne/cm, and the E2may be 10 to 100 dyne/cm.

For example, the E1 may be 10 to 80 dyne/cm, 10 to 60 dyne/cm, 10 to 40dyne/cm, or 10 to 20 dyne/cm. The E2 may be 10 to 80 dyne/cm, 10 to 60dyne/cm, 20 to 60 dyne/cm, to 60 dyne/cm, 30 to 50 dyne/cm, or 40 to 50dyne/cm.

In an embodiment, the adhesiveness index may be 3.5 or less.

Specifically, the adhesiveness index may be 3.0 or less, 2.5 or less,0.01 to 3.5, 0.01 to 3.0, 0.03 to 3.0, 0.05 to 3.0, 0.05 to 2.5, 0.1 to3, 0.1 to 2.5, 0.2 to 3.5, 0.2 to 3.0, 0.3 to 2.5 or 0.5 to 2.5, but itis not limited thereto.

If the above range is satisfied, the slip properties of the first sideand the second side can be enhanced.

The second side may be a side that has not been in direct contact withthe casting body (30) for casting the polyimide-based resin in theprocess for preparing the base film. That is, the second side may be anair side in contact with the air when the polyimide-based resin is cast.

Unlike the above, the second side may be a side that has been in directcontact with the casting body (30) in the process for preparing the basefilm. That is, the second side may be a belt side in contact with thecasting body, for example, a belt when the polyimide-based resin iscast.

The base film according to an embodiment is colorless, transparent, andenhanced in mechanical properties and optical properties such as haze,yellow index, and modulus.

According to an embodiment, the base film may have a haze of 3% or less.For example, the haze may be 2% or less, 1.5% or less, or 1% or less,but it is not limited thereto.

According to an embodiment, the base film may have a yellow index (YI)of 5 or less. For example, the yellow index may be 4 or less, 3.8 orless, 2.8 or less, 2.5 or less, 2.3 or less, or 2.1 or less, but it isnot limited thereto.

According to an embodiment, the base film may have a modulus of 5 GPa ormore. Specifically, the modulus may be 5.2 GPa or more, 5.5 GPa or more,6.0 GPa or more, 10 GPa or less, 5 GPa to 10 GPa, or 7 GPa to 10 GPa,but it is not limited thereto.

According to an embodiment, the base film may have a transmittance of80% or more. For example, the transmittance may be 85% or more, 88% ormore, 89% or more, 80% to 99%, 80% to 99%, or 85% to 99%, but it is notlimited thereto.

When the base film is perforated at a speed of 10 mm/min using a 2.5-mmspherical tip in a UTM compression mode, the maximum diameter (mm) ofperforation including a crack is 60 mm or less. Specifically, themaximum diameter of perforation may be 5 to 60 mm, 10 to 60 mm, 15 to 60mm, 20 to 60 mm, 25 to 60 mm, or 25 to 58 mm, but it is not limitedthereto.

The base film has a compressive strength is 0.4 kgf/μm or more.Specifically, the compressive strength may be 0.45 kgf/μm or more, or0.46 kgf/μm or more, but it is not limited thereto.

The base film has a surface hardness of HB or higher. Specifically, thesurface hardness may be H or higher, or 2H or higher, but it is notlimited thereto.

The base film has a tensile strength of 15 kgf/mm² or more.Specifically, the tensile strength may be 18 kgf/mm² or more, 20 kgf/mm²or more, 21 kgf/mm² or more, or 22 kgf/mm² or more, but it is notlimited thereto.

The base film has an elongation of 15% or more. Specifically, theelongation may be 16% or more, 17% or more, or 17.5% or more, but it isnot limited thereto.

The base film produced according to an embodiment has high oxidationresistance and can secure excellent optical properties such as highlight transmittance, low haze, and low yellow index (YI). Further, it ispossible to impart long-term stable mechanical properties to a substratethat requires flexibility in terms of modulus, elongation, tensilecharacteristics, and elastic restoring force.

Process for Preparing a Base Film (110)

FIG. 3 is a schematic flow diagram of a process for preparing a basefilm according to an embodiment.

Referring to FIG. 3, the process for preparing a base film comprisessimultaneously or sequentially mixing and reacting a diamine compound, adianhydride compound, and a dicarbonyl compound in an organic solvent ina polymerization apparatus to prepare a polymer solution (S100);transferring the polymer solution to a tank (S200); purging with aninert gas (S300); casting the polymer solution in the tank onto a beltand then drying it to prepare a gel-sheet (S400); thermally treating thegel-sheet while it is moved to prepare a cured film (S500); cooling thecured film while it is moved (S600); and winding the cooled cured filmusing a winder (S700).

In the process for preparing the base film (110), the polymer solutionis prepared by simultaneously or sequentially mixing and reacting adiamine compound, a dianhydride compound, and a dicarbonyl compound inan organic solvent in a polymerization apparatus (S100).

According to an embodiment, the polymer solution may be prepared bysimultaneously mixing and reacting the diamine compound, the dianhydridecompound, and the dicarbonyl compound in an organic solvent.

According to another embodiment, the step of preparing the polymersolution may comprise first mixing and reacting the diamine compound andthe dianhydride compound in a solvent to produce a polyamic acid (PAA)solution; and second mixing and reacting the polyamic acid (PAA)solution and the dicarbonyl compound to form an amide bond and an imidebond at the same time. The polyamic acid solution is a solution thatcomprises a polyamic acid.

According to still another embodiment, the step of preparing the polymersolution may comprise first mixing and reacting the diamine compound andthe dianhydride compound to produce a polyamic acid solution; subjectingthe polyamic acid solution to dehydration to produce a polyimide (PI)solution; and second mixing and reacting the polyimide (PI) solution andthe dicarbonyl compound to further form an amide bond. The polyimidesolution is a solution that comprises a polymer having an imide repeatunit.

According to another embodiment, the step of preparing the polymersolution may comprise first mixing and reacting the diamine compound andthe dicarbonyl compound to produce a polyamide (PA) solution; and secondmixing and reacting the polyamide solution and the dianhydride compoundto further form an imide bond. The polyamide solution is a solution thatcomprises a polymer having an amide repeat unit.

The polymer solution thus prepared may be a solution that comprises apolymer containing at least one selected from the group consisting of apolyamic acid (PAA) repeat unit, a polyamide (PA) repeat unit, and apolyimide (PI) repeat unit.

Alternatively, the polymer contained in the polymer solution comprisesan imide repeat unit derived from the polymerization of the diaminecompound and the dianhydride compound and an amide repeat unit derivedfrom the polymerization of the diamine compound and the dicarbonylcompound.

According to an embodiment, the step of preparing the polymer solutionmay further comprise introducing a catalyst.

The catalyst may include, for example, beta picoline and/or aceticanhydride, but it is not limited thereto. The further addition of thecatalyst may expedite the reaction rate and enhance the chemical bondingforce between the repeat units or that within the repeat units.

In an embodiment, the step of preparing the polymer solution may furthercomprise adjusting the viscosity of the polymer solution.

Specifically, the step of preparing the polymer solution may comprise(a) simultaneously or sequentially mixing and reacting a diaminecompound, a dianhydride compound, and a dicarbonyl compound in anorganic solvent to prepare a first polymer solution; (b) measuring theviscosity of the first polymer solution and evaluating whether thetarget viscosity has been reached; and (c) if the viscosity of the firstpolymer solution does not reach the target viscosity, further adding thedicarbonyl compound to prepare a second polymer solution having thetarget viscosity.

The target viscosity may be 100,000 cps to 500,000 cps at roomtemperature. Specifically, the target viscosity may be 100,000 cps to400,000 cps, 100,000 cps to 350,000 cps, 100,000 cps to 300,000 cps,150,000 cps to 300,000 cps, or 150,000 cps to 250,000 cps, but it is notlimited thereto.

In another embodiment, the content of solids contained in the polymersolution may be 10% by weight to 20% by weight. Specifically, thecontent of solids contained in the second polymer solution may be 12% byweight to 18% by weight, but it is not limited thereto.

If the content of solids contained in the polymer solution is within theabove range, a base film can be effectively produced in the extrusionand casting steps. In addition, the base film thus produced may havemechanical properties in terms of an improved modulus and the like andoptical properties in terms of a low yellow index and the like.

In an embodiment, the step of preparing the polymer solution may furthercomprise adjusting the pH of the polymer solution. In this step, the pHof the polymer solution may be adjusted to 4 to 7 or 4.5 to 7, but it isnot limited thereto.

The pH of the polymer solution may be adjusted by adding a pH adjustingagent. The pH adjusting agent is not particularly limited and mayinclude, for example, amine compounds such as alkoxyamine, alkylamine,and alkanolamine.

If the pH of the polymer solution is adjusted to the above range, it ispossible to prevent the damage to the equipment in the subsequentprocess, to prevent the occurrence of defects in the film produced fromthe polymer solution, and to achieve the desired optical properties andmechanical properties in terms of yellow index and modulus.

The pH adjusting agent may be employed in an amount of 0.1% by mole to10% by mole based on the total number of moles of monomers in thepolymer solution, but it is not limited thereto.

The step of preparing the polymer solution may further comprise purgingwith an inert gas. The step of purging with an inert gas may removemoisture, reduce impurities, increase the reaction yield, and impartexcellent surface appearance and mechanical properties to the filmfinally produced.

In such event, the inert gas may be at least one selected from the groupconsisting of nitrogen, helium (He), neon (Ne), argon (Ar), krypton(Kr), xenon (Xe), and radon (Rn), but it is not limited thereto.Specifically, the inert gas may be nitrogen.

The polyimide-based resin used to prepare the polymer solution may havea molar ratio of the imide repeat unit to the amide repeat unit of 20:80to 80:20, for example, 20:80 to 50:50. In such event, the imide repeatunit may be a repeat unit represented by the above Formula A, and theamide repeat unit may be a repeat unit represented by the above FormulaB.

If the molar ratio of the polyimide-based resin satisfies the aboverange, it is easy to control the viscosity of the polymer solution byusing the monomers as described above for preparing the same. As aresult, it is easy to produce a uniform film without defects on thesurface of the gel-sheet and the cured film.

It is possible to produce a polyimide-based film whose opticalproperties, mechanical properties, and flexibility are improved in awell-balanced manner without a complicated process by properlycontrolling the content of the imide repeat unit and that of the amiderepeat unit. In addition, it is possible to provide a polyimide-basedfilm whose optical properties, mechanical properties, and flexibilityare improved in a well-balanced manner without such steps asprecipitation, filtration, drying, and redissolution as adopted in theprior art. The content of the imide repeat unit and that of the amiderepeat unit may be controlled by the amounts of the aromatic dianhydrideand the dicarbonyl compound, respectively.

FIG. 4 schematically illustrates process facilities for preparing a basefilm according to an embodiment.

Specifically, the polymer solution as described above is prepared in apolymerization apparatus (10), and the polymer solution thus produced istransferred to, and stored in, a tank (20) (S200). Here, once thepolymer solution has been prepared, the step of transferring the polymersolution to the tank is carried out without any additional steps.

The polymer solution prepared in the polymerization apparatus istransferred to, and stored in, the tank without any separateprecipitation and redissolution steps for removing impurities. In theconventional process, in order to remove impurities such as hydrochloricacid (HCl) generated during the preparation of a polymer solution, thepolymer solution thus prepared is purified through a separate step toremove the impurities, and the purified polymer solution is thenredissolved in a solvent. In this case, however, there has been aproblem that the loss of the active ingredient increases in the step ofremoving the impurities, resulting in decreases in the yield.

Accordingly, the preparation process according to an embodimentultimately minimizes the amount of impurities generated in the step ofpreparing the polymer solution or properly controls the impurities inthe subsequent steps, even if a certain amount of impurities is present,so as not to deteriorate the physical properties of the final film.Thus, the process has an advantage in that a film is produced withoutseparate precipitation or redissolution steps. In addition, the polymersolution is not ought to be subjected to such separate steps asprecipitation, filtration, drying, and redissolution. Since the solutionproduced in the polymerization step can be directly applied to thecasting step, the yield can be remarkably enhanced.

The tank (20) is a place for storing the polymer solution before formingit into a film, and its internal temperature may be −20° C. to 20° C.

Specifically, the internal temperature may be −20° C. to 15° C., −20° C.to 10° C., −20° C. to 5° C., or −20° C. to 0° C., but it is not limitedthereto.

If the temperature of the tank (20) is controlled to the above range, itis possible to prevent the polymer solution from deteriorating duringstorage, and it is possible to lower the moisture content to therebyprevent defects of the film produced therefrom.

The process for preparing a base film may further comprise carrying outvacuum degassing of the polymer solution transferred to the tank (20).

The vacuum degassing may be carried out for 30 minutes to 3 hours afterdepressurizing the internal pressure of the tank to 0.1 to 0.7 bar. Thevacuum degassing under these conditions may reduce bubbles in thepolymer solution. As a result, it is possible to prevent surface defectsof the film produced therefrom and to achieve excellent opticalproperties such as haze.

In addition, the process for preparing a polyimide-based film mayfurther comprise purging the polymer solution transferred to the tank(20) with an inert gas (S300).

Specifically, the purging is carried out by purging the tank with aninert gas at an internal pressure of 1 atm to 2 atm. The nitrogenpurging under these conditions may remove moisture in the polymersolution, reduce impurities to thereby increase the reaction yield, andachieve excellent optical properties such as haze and mechanicalproperties.

The step of vacuum degassing and the step of purging the tank withnitrogen gas are performed in a separate process, respectively.

For example, the step of vacuum degassing may be carried out, followedby the step of purging the tank with nitrogen gas, but it is not limitedthereto.

The step of vacuum degassing and/or the step of purging the tank withnitrogen gas may improve the physical properties of the surface of thepolyimide-based film thus produced.

Thereafter, the process may further comprise storing the polymersolution in the tank (20) for 12 hours to 360 hours. Here, thetemperature inside the tank may be kept at −20° C. to 20° C.

The process for preparing a base film may further comprise casting thepolymer solution in the tank and then drying it to prepare a gel-sheet(S400).

The polymer solution may be cast onto a casting body such as a castingroll or a casting belt.

Referring to FIG. 4, in an embodiment, the polymer solution may beapplied onto a casting belt (30) as a casting body, and it is driedwhile it is moved to be made into a sheet in the form of a gel.

When the polymer solution is injected onto the belt (30), the injectionamount may be 300 g/min to 700 g/min. If the injection amount of thepolymer solution satisfies the above range, the gel-sheet can beuniformly formed to an appropriate thickness.

In addition, the casting thickness of the polymer solution may be 200 to700 μm. If the polymer solution is cast to a thickness within the aboverange, the final film produced after the drying and thermal treatmentmay have an appropriate and uniform thickness.

The polymer solution is cast and then dried at a temperature of 60° C.to 150° C. for 5 minutes to 60 minutes to prepare a gel-sheet. Thesolvent of the polymer solution is partially or totally volatilizedduring the drying to prepare the gel-sheet.

As described above, the viscosity of the polymer solution at roomtemperature may be 100,000 cps to 500,000 cps, for example, 100,000 cpsto 400,000 cps, for example, 100,000 cps to 350,000 cps, for example,150,000 cps to 350,000 cps. If the viscosity satisfies the above range,the polymer solution can be cast onto a belt in a uniform thicknesswithout defects.

The process for preparing a base film comprises thermally treating thegel-sheet while it is moved to prepare a cured film (S500).

Referring to FIG. 4, the thermal treatment of the gel-sheet may becarried out by passing it through a thermal curing device (40).

The thermal treatment of the gel-sheet may be carried out in atemperature range of 80° C. to 500° C. at a temperature elevation rateof 2° C./min to 80° C./min for 5 minutes to 40 minutes. Specifically,the thermal treatment of the gel-sheet may be carried out in atemperature range of 80° C. to 470° C. at a temperature elevation rateof 10° C./min to 80° C./min for 5 minutes to 30 minutes, but it is notlimited thereto.

In such event, the initial temperature of the thermal treatment of thegel-sheet may be 80° C. or higher, and the maximum temperature in thethermal treatment may be 300° C. to 500° C. For example, the maximumtemperature in the thermal treatment may be 350° C. to 500° C., 380° C.to 500° C., 400° C. to 500° C., 410° C. to 480° C., 410° C. to 470° C.,or 410° C. to 450° C.

That is, referring to FIG. 4, the inlet temperature of the thermalcuring device (40) may be the initial temperature of the thermaltreatment, and the temperature of a certain region inside the thermalcuring device (40) may be the maximum temperature in the thermaltreatment.

The thermal treatment under these conditions may cure the gel-sheet tohave appropriate surface hardness and modulus and may secure high lighttransmittance and low haze of the cured film at the same time.

The process for preparing a base film comprises cooling the cured filmwhile it is moved (S600).

Referring to FIG. 4, the cooling of the cured film is carried out afterit has been passed through the thermal curing device (40). It may becarried out by using a separate cooling chamber (not shown) or byforming an appropriate temperature atmosphere without a separate coolingchamber.

The step of cooling the cured film while it is moved may comprise afirst temperature lowering step of reducing the temperature at a rate of100° C./min to 1,000° C./min and a second temperature lowering step ofreducing the temperature at a rate of 40° C./min to 400° C./min.

In such event, specifically, the second temperature lowering step isperformed after the first temperature lowering step. The temperaturelowering rate of the first temperature lowering step may be faster thanthe temperature lowering rate of the second temperature lowering step.

For example, the maximum rate of the first temperature lowering step isfaster than the maximum rate of the second temperature lowering step.Alternatively, the minimum rate of the first temperature lowering stepis faster than the minimum rate of the second temperature lowering step.

If the step of cooling the cured film is carried in such a multistagemanner, it is possible to have the physical properties of the cured filmfurther stabilized and to maintain the optical properties and mechanicalproperties of the film achieved during the curing step more stably for along period of time.

The moving speed of the gel-sheet and the moving speed of the cured filmare the same.

The process for preparing a base film comprises winding the cooled curedfilm using a winder (S700).

Referring to FIG. 4, the cooled cured film may be wound using aroll-shaped winder (50).

In such event, the ratio of the moving speed of the gel-sheet on thebelt at the time of drying to the moving speed of the cured film at thetime of winding is 1:0.95 to 1:1.40. Specifically, the ratio of themoving speeds may be 1:0.99 to 1:1.20, 1:0.99 to 1:1.10, 1:1.01 to1:1.10, or 1:1.05 to 1:1.10, but it is not limited thereto.

If the ratio of the moving speeds is outside the above range, themechanical properties of the cured film may be impaired, and theflexibility and elastic properties may be deteriorated.

Specifically, the moving speed of the gel-sheet on the belt at the timeof drying may be 0.1 m/min to 15 m/min, for example, 0.5 m/min to 10m/min.

In the process for preparing a base film, the thickness variation (%)according to the following Relationship 1 may be 3% to 30%, for example,5% to 20%, but it is not limited thereto.

Thickness variation (%)=(M1−M2)/M2×100  [Relationship 1]

In Relationship 1, M1 is the thickness (μm) of the gel-sheet, and M2 isthe thickness (μm) of the cooled cured film at the time of winding.

The physical properties of the base film as described above are based ona thickness of 40 μm to 60 μm. For example, the physical properties ofthe base film are based on a thickness of 50 μm. In addition, the “MDdirection” refers to the direction in which the belt moves during thepreparation of the film, and the “TD direction” refers to the directionperpendicular to the MD direction.

The base film prepared by the preparation process as described above isexcellent in optical properties and mechanical properties. The base filmmay be applicable to various uses that require flexibility andtransparency. For example, the base film may be applied to solar cells,displays, semiconductor devices, sensors, and the like.

Functional Layer (120)

The functional layer (120) may comprise an organic resin, an inorganicfiller, and other additives.

The organic resin may be a curable resin. The organic resin may be abinder resin. The organic resin may be at least one selected from thegroup consisting of an acrylate-based monomer, a urethane acrylate-basedoligomer, and an epoxy acrylate-based oligomer.

The acrylate-based monomer may be at least one selected from the groupconsisting of a substituted or unsubstituted acrylate and a substitutedor unsubstituted methacrylate.

The acrylate-based monomer may contain 1 to 10 functional groups. Theurethane acrylate-based oligomer may contain 2 to 15 functional groups.The epoxy acrylate-based oligomer may contain 1 to 10 functional groups.

Examples of the acrylate-based monomers include trimethylolpropanetriacrylate (TMPTA), trimethylolpropaneethoxy triacrylate (TMPEOTA),glycerin propoxylated triacrylate (GPTA), pentaerythritol tetraacrylate(PETA), and dipentaerythritol hexaacrylate (DPHA).

Examples of the urethane acrylate-based oligomer include a bifunctionalurethane acrylate oligomer having a weight average molecular weight of1,400 to 25,000, a trifunctional urethane acrylate oligomer having aweight average molecular weight of 1,700 to 16,000, a tetra-functionalurethane acrylate oligomer having a weight average molecular weight of1000, a hexa-functional urethane acrylate oligomer having a weightaverage molecular weight of 818 to 2,600, an ennea-functional urethaneacrylate oligomer having a weight average molecular weight of 3,500 to5,500, a deca-functional urethane acrylate oligomer having a weightaverage molecular weight of 3,200 to 3,900, and apentakaideca-functional urethane acrylate oligomer having a weightaverage molecular weight of 2,300 to 20,000.

Examples of the epoxy acrylate-based oligomer include a monofunctionalepoxy acrylate oligomer having a weight average molecular weight of 100to 300, a bifunctional epoxy acrylate oligomer having a weight averagemolecular weight of 250 to 2,000, and a tetra-functional epoxy acrylateoligomer having a weight average molecular weight of 1,000 to 3,000.

The acrylate-based monomer may have a weight average molecular weight(Mw) of about 200 to about 2,000 g/mole, about 200 to about 1,000g/mole, or about 200 to about 500 g/mole.

The acrylate equivalent weight of the acrylate-based monomer may rangefrom about 50 to about 300 g/eq, from about 50 to about 200 g/eq, orfrom about 50 to about 150 g/eq.

The epoxy equivalent weight of the epoxy acrylate-based oligomer mayrange from about 50 to about 300 g/eq, from about 50 to about 200 g/eq,or from about 50 to about 150 g/eq.

The content of the organic resin may be 30% by weight to 100% by weightbased on the total weight of the functional layer. Specifically, thecontent of the organic resin may be 40% by weight to 90% by weight, or50% by weight to 80% by weight, based on the total weight of thefunctional layer.

Examples of the inorganic filler include silica, barium sulfate, zincoxide, and alumina.

The inorganic filler may have a particle diameter of 1 nm to 100 nm.Specifically, the particle diameter of the inorganic filler may be 5 nmto 50 nm or 10 nm to 30 nm.

The inorganic filler may comprise inorganic fillers having particle sizedistributions different from each other. For example, the inorganicfiller may comprise a first inorganic filler having a d50 of 20 to 35 nmand a second inorganic filler having a d50 of 40 to 130 nm.

The content of the inorganic filler may be about 25% by weight or more,about 30% by weight or more, or about 35% by weight or more, based onthe total weight of the functional layer. In addition, the content ofthe inorganic filler may be about 50% by weight or less, about 45% byweight or less, or about 40% by weight or less, based on the totalweight of the functional layer.

The inorganic filler may be subjected to surface treatment. Theinorganic filler may be subjected to surface treatment with a silanecoupling agent or the like. Examples of the silane coupling agentinclude (meth)acrylsilane, methacroxysilane, vinylsilane, epoxysilane,and mercaptosilane.

The functional layer may further comprise a photoinitiator.

Examples of the photoinitiator include 1-hydroxy-cyclohexyl-phenylketone, 2-hydroxy-2-methyl-1-phenyl-1-propanone,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone,methylbenzoylformate, α,α-dimethoxy-a-phenylacetophenone,2-benzoyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone,2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone,diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide, andbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, but it is not limitedthereto. In addition, commercially available products include Irgacure184, Irgacure 500, Irgacure 651, Irgacure 369, Irgacure 907, Darocur1173, Darocur MBF, Irgacure 819, Darocur TPO, Irgacure 907, and EsacureKIP 100F. The photoinitiator may be used alone or in combination of twoor more different types.

The functional layer may comprise a surfactant, a UV absorber, a UVstabilizer, an anti-yellowing agent, a leveling agent, an antifoulingagent, or a dye for improving chromaticity values as other additives. Inaddition, the content of the additives may be variously adjusted withina range that does not deteriorate the physical properties of thefunctional layer. For example, the content of the additives may be about0.01% by weight to about 10% by weight based on the total weight of thefunctional layer, but it is not limited thereto.

The surfactant may be a mono- to bifunctional fluorine-based acrylate, afluorine-based surfactant, or a silicone-based surfactant. Thesurfactant may be employed in a form dispersed or crosslinked in thefunctional layer.

Examples of the UV absorber include benzophenone-based compounds,benzotriazole-based compounds, and triazine-based compounds. Examples ofthe UV stabilizer include tetramethyl piperidine and the like.

A coating composition may be prepared in order to form the functionallayer. The coating composition comprises the organic resin, theinorganic filler, the additives, and an organic solvent.

Examples of the organic solvent include alcohol-based solvents such asmethanol, ethanol, isopropyl alcohol, and butanol; alkoxy alcohol-basedsolvents such as 2-methoxyethanol, 2-ethoxyethanol, and1-methoxy-2-propanol; ketone-based solvents such as acetone, methylethyl ketone, methyl isobutyl ketone, methyl propyl ketone, andcyclohexanone; ether-based solvent such as propylene glycol monopropylether, propylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monopropyl ether, ethylene glycol monobutylether, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol monopropyl ether, diethylene glycol monobutylether, and diethylene glycol-2-ethylhexyl ether; and aromatic solventssuch as benzene, toluene, and xylene, which may be used alone or incombination thereof.

The content of the organic solvent is not particularly limited since itmay be variously adjusted within a range that does not deteriorate thephysical properties of the coating composition. The organic solvent maybe employed such that the weight ratio of the solids content of thecomponents contained in the coating composition to the organic solventmay be about 30:70 to about 99:1. If the content of the organic solventis within the above range, the composition may have appropriate fluidityand coatability.

Since the organic solvent is used in the course of preparing thefunctional layer, a trace amount of the organic solvent may remain inthe functional layer.

The coating composition may be applied to the front or rear side of thebase film. The coating composition may be coated by a bar coatingmethod, a knife coating method, a roll coating method, a blade coatingmethod, a die coating method, a microgravure coating method, a commacoating method, a slot die coating method, a lip coating method, or asolution casting method.

Thereafter, the organic solvent contained in the coating composition maybe removed. The organic solvent may be removed by evaporation.

Thereafter, the coating composition layer may be cured by light and/orheat.

The functional layer upon complete curing thereof may have a thicknessof about 2 μm or more, or about 3 μm or more, for example, about 2 toabout 20 μm, about 2 to about 15 μm, about 2 to about 10 μm, or about 3to about 10 μm. An additional layer may be further disposed between thebase film and the functional layer. The additional layer may be anantistatic layer, which performs an antistatic function, or may be a lowrefractive index layer, which performs a low reflection function.Alternatively, the functional layer itself may perform an antistaticfunction and/or a low reflection function.

Physical Properties of the Polyimide-Based Composite Film

The polyimide-based composite film according to an embodiment has anadhesiveness index represented by the following Equation 1 of less than10 or less.

$\begin{matrix}{{{Adhesiveness}\mspace{14mu} {index}} = \frac{{Fc} \times {Ec}}{Sc}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

In Equation 1, Sc is (Sz1+Sz2)/2 μm, Fc is (F1+F2)/0.6, and Ec is(E1+E2)/40 dyne/cm, Sz1 is the Sz roughness (μm) of the first side, Sz2is the Sz roughness (μm) of the second side, F1 is the coefficient ofstatic friction between the first side and the second side, F2 is acoefficient of kinetic friction between the first side and the secondside, E1 is the surface energy (dyne/cm) of the first side, and the E2is the surface energy (dyne/cm) of the second side.

The ranges of the adhesiveness index, Sz1, Sz2, F1, F2, E1, and E2 areas described above.

The polyimide-based composite film according to an embodiment satisfiesthe above range of the surface roughness, coefficients of friction, andsurface energies of the first side and the second side, so that the slipproperties of the first side and second side can be enhanced.

In particular, when the polyimide-based composite film according to anembodiment is wound, the first side and the second side may be incontact with each other. In such event, as the first side and the secondside have sufficient slip properties, the problem caused by the frictionbetween the first side and the second side may be reduced.

Accordingly, the polyimide-based composite film according to anembodiment not only can effectively prevent the problem caused by thefirst side and the second side being excessively adhered to each other,but also is colorless and transparent with enhanced mechanicalproperties and optical properties such as haze, yellow index, andmodulus.

In an embodiment, the polyimide-based composite film may have a haze of3% or less, a yellow index (YI) of 5 or less, a modulus of 5 GPa ormore, and a transmittance of 80% or more.

Display Device

The display device according to another embodiment comprises a displaypanel; and a cover window disposed on the display panel, wherein thecover window comprises a base film and a functional layer disposed onthe base film, and the base film comprises a polyimide-based resin. Inaddition, when the side of the functional layer located opposite to theside in contact with the base film is referred to as a first side andwhen the side of the base film located opposite to the side in contactwith the functional layer is referred to as a second side, theadhesiveness index represented by the above Equation 1 is 3.5 or less.

Here, the details on the base film, the functional layer, and the likeare as described above.

FIG. 2 is a cross-sectional view of a display device according to anembodiment. Specifically, FIG. 2 illustrates a cross-section of adisplay device, which comprises a display panel (300) and a cover window(100) disposed on the display panel (300), wherein the cover window(100) comprises a base film (110) and a functional layer (120), and anadhesive layer (200) is interposed between the display panel (300) andthe cover window (100). The functional layer (120) is disposed on theviewing side with respect to the base film (110).

The display panel (300) is for displaying an image, and it may haveflexible characteristics.

The display panel (300) may be a display panel for displaying an image.For example, it may be a liquid crystal display panel or an organicelectroluminescent display panel. Specifically, the organicelectroluminescent display panel may comprise a front polarizing plateand an organic EL panel, but it is not limited thereto.

The front polarizing plate may be disposed on the front side of theorganic EL panel. Specifically, the front polarizing plate may beattached to the side on which an image is displayed in the organic ELpanel.

The organic EL panel displays an image by self-emission of a pixel unit.The organic EL panel may comprise an organic EL substrate and a drivingsubstrate. The organic EL substrate may comprise a plurality of organicelectroluminescent units, each of which corresponds to a pixel.Specifically, it may comprise a cathode, an electron transport layer, alight-emitting layer, a hole transport layer, and an anode. The drivingsubstrate is operatively coupled to the organic EL substrate. That is,the driving substrate may be coupled to the organic EL substrate so asto apply a driving signal such as a driving current, so that the drivingsubstrate can drive the organic EL substrate by applying a current tothe respective organic electroluminescent units.

According to an embodiment, an adhesive layer (200) may be interposedbetween the display panel (300) and the cover window (100). The adhesivelayer (200) may be an optically transparent adhesive layer, but it isnot particularly limited.

The cover window (100) is disposed on the display panel (300). The coverwindow (100) is located at the outermost position of the display deviceaccording to an embodiment to thereby protect the display panel (300).

The cover window (100) may comprise the base film (110) and thefunctional layer (120). The functional layer (120) may be at least oneselected from the group consisting of a hard coating layer, areflectance reducing layer, an antifouling layer, and an antiglarelayer. The functional layer (120) may be coated on at least one side ofthe base film (110).

Hereinafter, the above description will be described in detail byreferring to examples. But the following Examples are intended toillustrate the present invention, and the scope of the Examples is notlimited thereto only.

EXAMPLE Example 1

A 1,000-liter glass reactor equipped with a temperature-controllabledouble jacket was charged with 250 kg of dimethylacetamide (DMAc) as anorganic solvent at 20° C. under a nitrogen atmosphere. Then, 32.02 kg of2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) as an aromaticdiamine was slowly added thereto and dissolved.

Subsequently, while 13.3 kg of2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA) as anaromatic dianhydride was slowly added thereto, the mixture was stirredfor 1 hour.

Then, 700 g of barium sulfate as a filler was added thereto, followed bystirring for 1 hour.

Then, 12.56 kg of 1,1′-biphenyl-4,4′-dicarbonyldichloride (BPDC) as afirst dicarbonyl compound was added, followed by stirring for 1 hour.And 4.77 kg, which was 94% based on the introduced molar amount, ofterephthaloyl chloride (TPC) as a second dicarbonyl compound was added,followed by stirring for 1 hour, thereby preparing a first polymersolution.

The viscosity of the first polymer solution thus prepared was measured.If the measured viscosity did not reach the target viscosity, a TPCsolution in a DMAc organic solvent at a concentration of 10% by weightwas prepared, and 1 ml of the TPC solution was added to the firstpolymer solution, followed by stirring the mixture for 30 minutes. Thisprocedure was repeated until the viscosity became 200,000 cps, therebypreparing a second polymer solution.

The second polymer solution was transferred to a tank and stored at −10°C. It was degassed for 1.5 hours so that the pressure in the tankreached 0.3 bar. The tank was purged with nitrogen gas at an internalpressure of 1.5 atm. Upon the purging, the second polymer solution wasstored in the tank for 30 hours.

Subsequently, the second polymer solution was cast onto astainless-steel belt and then dried with hot air at 80° C. for 30minutes, thereby producing a gel-sheet. Then, while the gel-sheet wasmoved, it was heated in a temperature range of 80° C. to 350° C. at atemperature elevation rate of 2° C./min to 80° C./min, followed bythermal treatment at the highest temperature for about 25 minutes.Thereafter, a first temperature lowering step was carried out byreducing the temperature at a rate of about 800° C./min, followed by asecond temperature lowering step by reducing the temperature at a rateof about 100° C./min, thereby obtaining a base film, which was woundusing a winder. In such event, the moving speed of the gel-sheet on thebelt at the time of drying was 1 m/s. The speed of the winder wascontrolled such that the ratio of the moving speed of the gel-sheet onthe belt at the time of drying to the moving speed of the film at thetime of winding was within the range of 1:1.01 to 1:1.10.

A hard-coating layer was formed on one side of the polyimide-based filmthus prepared. In order to form the hard-coating layer, 12 parts byweight of a multifunctional acrylate (M600, Miwon Specialty Chemical),20 parts by weight of a urethane acrylate (PU2050, Miwon SpecialtyChemical), 8 parts by weight of a nano-silica sol (average particlediameter: 12 nm), 60 parts by weight of methyl isobutyl ketone, 0.8parts by weight of a photoinitiator (Irgacure 184, BASF), and 0.2 partsby weight of a leveling agent (601ADH2, Neos) were compounded with astirrer to prepare a composition for forming a hard coating. Thereafter,the mixed coating composition was coated by a slot die coating method toa thickness of about 5 μm on one side of the polyimide-based film.Thereafter, the coated composition was dried at about 90° C. for about 2minutes and cured by UV at 600 mJ/cm². As a result, a polyimide-basedcomposite film comprising a polyimide-based film and a hard-coatinglayer was prepared.

Examples 2 and 3 and Comparative Examples 1 and 2

Tests were performed in the same manner as in Example 1, except that thecontents of the respective reactants, the content of the filler, theparticle diameter of the inorganic particles, and the maximumtemperature and time of the thermal treatment were changed as shown inTable 1 below. In addition, tests were performed in the same manner asin Example 1 with respect to the formation of a hard-coating layer,except that the contents of the multifunctional acrylate, urethaneacrylate, nano-silica sol, and leveling agent (KY1203 Shin-Etsu orDAC-HP Daikin) were changed as shown in Table 2 below.

TABLE 1 Filler Filler Max. temp. Time of TFDB 6FDA TPC BPDC contentparticle of thermal thermal (molar (molar (molar (molar (% by diametertreatment treatment ratio) ratio) ratio) ratio) weight) (μm) (° C.)(min.) Ex. 1 0.20 0.06 0.05 0.09 1.1 0.1 350 25 Ex. 2 0.20 0.05 0.060.09 1.1 0.1 300 20 Ex. 3 0.20 0.03 0.10 0.07 0.6 0.1 350 25 C. Ex. 10.20 0.06 0.05 0.09 0.6 1.0 250 20 C. Ex. 2 0.20 0.05 0.06 0.09 — — 25020

TABLE 2 Urethane Multifunctional acrylate acrylate Nano-silica Levelingagent PU2050 M600 sol 601ADH2 KY1203 DAC-HP Ex. 1 20 12 8 0.2 — — Ex. 220 12 8 — 0.2 — Ex. 3 20 12 8 — — 0.2 C. Ex. 1 20 12 8 — — — C. Ex. 2 2515 — — — —

Evaluation Example

The films prepared in Examples 1 to 3 and Comparative Examples 1 and 2were each measured and evaluated for the following properties.

Evaluation Example 1: Measurement of Surface Energy

It was measured according to the German industrial standard (DIN 55660)using a mobile surface analyzer from Kruss in Germany.

Evaluation Example 2: Measurement of Friction Coefficient

Two polyimide-based composite films cut to a size of 10 cm-10 cm wereprepared. The coefficient of static friction and the coefficient ofkinetic friction between the first side and the second side weremeasured according to the standard method of ASTM D1894 using a frictioncoefficient meter from Qmesys in Korea.

Evaluation Example 3: Measurement of Surface Roughness

The Sz roughness was measured using a 3D optical profiler contour GTfrom Bruker. The Sz roughness of the surface was measured at arbitrarythree locations on the films of Examples and Comparative Examples, andan average value thereof was obtained.

An image was taken by the 3D optical profiler in the region of 220 μm220 μm at each location, and the Sz roughness was measured therefrom.When the roughness was measured, Sz is a value defined according to ISO25178-2:2012. Sz is the maximum height and is the sum of maximum peakheight (Sp) and maximum pit height (Sv).

Evaluation Example 4: Measurement of Adhesiveness Index

The adhesiveness index was calculated according to the followingEquation 1.

$\begin{matrix}{{{Adhesiveness}\mspace{14mu} {index}} = \frac{{Fc} \times {Ec}}{Sc}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, Sc is (Sz1+Sz2)/2 μm, Fc is (F1+F2)/0.6, and Ec is (E1+E2)/40dyne/cm, wherein Sz1 is the Sz roughness (μm) of the first side, Sz2 isthe Sz roughness (μm) of the second side, F1 is the coefficient ofstatic friction between the first side and the second side, F2 is acoefficient of kinetic friction between the first side and the secondside, E1 is the surface energy (dyne/cm) of the first side, and the E2is the surface energy (dyne/cm) of the second side.

The results of Evaluation Examples 1 to 4 are shown in Table 3 below.

TABLE 3 Surface Surface energy energy Coefficient Coefficient RoughnessRoughness of second of first of static of kinetic of second of firstAdhesiveness side side Ec friction friction Fc side side Sc index Ex. 140 11 1.28 0.44 0.15 0.98 1.7 0.37 1.04 1.21 Ex. 2 42 14 1.40 0.46 0.171.05 1.1 0.38 0.74 1.99 Ex. 3 41 18 1.48 0.43 0.18 1.02 1.3 0.35 0.831.82 C. Ex. 1 39 30 1.73 0.54 0.42 1.60 0.9 0.54 0.72 3.84 C. Ex. 2 3840 1.95 0.55 0.51 1.77 0.6 0.55 0.58 5.95

As shown in Table 3 above, the composite films prepared in Examples 1 to3 had an adhesiveness index with enhanced slip properties as comparedwith the composite films prepared in Comparative Examples 1 and 2.Specifically, the films of Comparative Examples 1 and 2 had too high anadhesiveness index, so that they had low slip properties, which maycause the defect that they could not be easily separated upon completionof the preparation process.

Evaluation Example 5: Measurement of Film Thickness

The thickness was measured at 5 points in the transverse direction usinga digital micrometer 547-401 manufactured by Mitutoyo Corporation. Theiraverage value was adopted as the thickness.

Evaluation Example 6: Measurement of Haze

The haze was measured using a haze meter NDH-5000W manufactured byNippon Denshoku Kogyo.

Evaluation Example 7: Measurement of Modulus

A sample was cut out by at least 5 cm in the direction perpendicular tothe main shrinkage direction of the film and by 10 cm in the mainshrinkage direction. It was fixed by the clips disposed at intervals of5 cm in a universal testing machine UTM 5566A of Instron. Astress-strain curve was obtained until the sample was fractured while itwas stretched at a rate of 5 mm/min at room temperature. The slope ofthe load with respect to the initial strain on the stress-strain curvewas taken as the modulus (GPa).

Evaluation Example 8: Measurement of Yellow Index

The yellow Index (YI) was measured with a spectrophotometer (UltraScanPRO, Hunter Associates Laboratory) using a CIE colorimetric system.

Evaluation Example 9: Measurement of Transmittance

The transmittance at 550 nm was measured using a haze meter NDH-5000Wmanufactured by Nippon Denshoku Kogyo.

Evaluation Example 10: Adhesiveness Test

Two polyimide-based composite films cut to a size of 10 cm×10 cm wereprepared. The two polyimide-based composite films were superposed withthe first and second sides in contact with each other, which was thencompressed at a temperature of 40° C. for 24 hours under a load of 20kg. Thereafter, the compressed polyimide-based composite film was cooledto room temperature. If the lower film was detached by its own weight,it was marked as ∘. If it was not detached, it was marked as x.

Evaluation Example 11: Scratch Test

The polyimide-based composite film in a roll shape was taken three timesat 1 m each and observed with the naked eyes. If longitudinal ortransverse scratches observed were less than 10, it was marked as ∘. Ifthey were 10 or more, it was marked as x.

The results of Evaluation Examples 5 to 11 are shown in Tables 4 below.

TABLE 4 Thickness (base layer/ Adhesiveness hard-coating) Haze ModulusYI Transmittance test Scratch Ex. 1 50/5 0.41 5.72 2.1 91 ∘ ∘ Ex. 2 50/50.42 5.74 2.2 92 ∘ ∘ Ex. 3 50/5 0.43 5.73 2.3 91 ∘ ∘ C. Ex. 1 50/5 0.455.71 2.1 90 x x C. Ex. 2 50/5 0.38 5.3 2.3 91 x x

As shown in Table 4 above, the films prepared in Examples 1 to 3 had alow adhesiveness index with enhanced slip properties as compared withthe films prepared in Comparative Examples 1 and 2. They also hadexcellent mechanical properties and optical properties such as haze,modulus, yellow index, and transmittance in balance.

REFERENCE NUMERALS OF THE DRAWINGS

-   -   10: polymerization apparatus    -   20: tank    -   30: casting body    -   40: thermal curing device    -   50: winder    -   100: cover window    -   101: first side    -   102: second side    -   110: base film    -   120: functional layer    -   200: adhesive layer    -   300: display panel    -   400: display device

1. A polyimide-based composite film, which comprises a base filmcomprising a polyimide-based resin; and a functional layer disposed onthe base film, wherein when the side of the functional layer locatedopposite to the side in contact with the base film is referred to as afirst side and when the side of the base film located opposite to theside in contact with the functional layer is referred to as a secondside, the adhesiveness index represented by the following Equation 1 is3.5 or less: $\begin{matrix}{{{Adhesiveness}\mspace{14mu} {index}} = \frac{{Fc} \times {Ec}}{Sc}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$ wherein Sc is (Sz1+Sz2)/2 μm, Fc is (F1+F2)/0.6, Ec is(E1+E2)/40 dyne/cm, Sz1 is the Sz roughness (μm) of the first side, Sz2is the Sz roughness (μm) of the second side, F1 is the coefficient ofstatic friction between the first side and the second side, F2 is acoefficient of kinetic friction between the first side and the secondside, E1 is the surface energy (dyne/cm) of the first side, and the E2is the surface energy (dyne/cm) of the second side.
 2. Thepolyimide-based composite film of claim 1, wherein the Sz1 is 0.01 μm to5 μm, and Sz2 is 0.01 μm to 5 μm.
 3. The polyimide-based composite filmof claim 1, wherein the Sz1 is 0.01 μm to 4 μm, and Sz2 is 0.01 μm to 4μm.
 4. The polyimide-based composite film of claim 1, wherein the Sz1 is0.01 μm to 3 μm, and Sz2 is 0.01 μm to 3 μm.
 5. The polyimide-basedcomposite film of claim 1, wherein the F1 is 0.1 to 0.8, and the F2 is0.1 to 0.8.
 6. The polyimide-based composite film of claim 1, whereinthe E1 is 10 to 100 dyne/cm, and the E2 is 10 to 100 dyne/cm.
 7. Thepolyimide-based composite film of claim 1, wherein the adhesivenessindex is or less.
 8. The polyimide-based composite film of claim 1,wherein the adhesiveness index is 0.01 to
 3. 9. The polyimide-basedcomposite film of claim 1, wherein the base film further comprises afiller.
 10. The polyimide-based composite film of claim 9, wherein thefiller is at least one selected from the group consisting of bariumsulfate, silica, and calcium carbonate.
 11. The polyimide-basedcomposite film of claim 9, wherein the filler has a particle diameter of0.01 μm to less than 1.0 μm.
 12. The polyimide-based composite film ofclaim 9, wherein the base film comprises the filler in an amount of 0.01to 3% by weight based on the total weight of the base film.
 13. Thepolyimide-based composite film of claim 1, which a haze of 3% or less, ayellow index of 5 or less, a modulus of 5 GPa or more, and atransmittance of 80% or more.
 14. A display device, which comprises adisplay panel; and a cover window disposed on the display panel, whereinthe cover window comprises a base film; and a functional layer disposedon the base film, the base film comprises a polyimide-based resin, andwhen the side of the functional layer located opposite to the side incontact with the base film is referred to as a first side and when theside of the base film located opposite to the side in contact with thefunctional layer is referred to as a second side, the adhesiveness indexrepresented by the following Equation 1 is 3.5 or less: $\begin{matrix}{{{Adhesiveness}\mspace{14mu} {index}} = \frac{{Fc} \times {Ec}}{Sc}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$ wherein Sc is (Sz1+Sz2)/2 μm, Fc is (F1+F2)/0.6, Ec is(E1+E2)/40 dyne/cm, Sz1 is the Sz roughness (μm) of the first side, Sz2is the Sz roughness (μm) of the second side, F1 is the coefficient ofstatic friction between the first side and the second side, F2 is acoefficient of kinetic friction between the first side and the secondside, E1 is the surface energy (dyne/cm) of the first side, and the E2is the surface energy (dyne/cm) of the second side.